Two days ago, the last male northern white rhino (Ceratotherium simum cottoni) died. His passing leaves two surviving members of his subspecies: both females who are unable to bear calves.

Even though it might not be quite the end of the northern white rhino because of the possibility of implanting frozen embryos in their southern cousins (C. simum simum), in practical terms, it nevertheless represents the end of a long decline for the subspecies. It also raises the question: how many individuals does a species need to persist?

No inbreeding, please

The Red List’s assessment criteria are based on the so-called “50/500 rule”. This states that to avoid inbreeding depression (the loss of “fitness” due to genetic problems), an effective population size of at least 50 individuals in a population is required.

To avoid eroding evolutionary potential (the ability of a population to evolve to cope with future environmental changes), an effective population of at least 500 is required.

The key here is that little qualifier “effective”. This refers to individuals who can breed with each other without causing inbreeding or loss of genetic diversity. A family unit, for example, might have only one or two reproductively effective members. But they would also need another, unrelated, family unit nearby for their offspring to reproduce with.

That means that the number of effective individuals is lower than the total population. On average, the ratio is about 0.1 to 0.2; that is, one effective individual (genetically speaking) for every five to ten members of the population.

This also assumes that the breeding pairs are matching up based on an optimal genetic basis – what geneticists call an “idealised population”.

In a perfect world, a breeding pair of animals would be completely unrelated and would have no chance of producing babies with any genetic defects caused by inbreeding. However, real populations rarely behave like this, so some pairs have a certain amount of relatedness. As the population gets smaller, the chance of breeding with a relative increases, which leads to more frequent and severe inbreeding.

Repopulating the world after the apocalypse

So let’s do the maths. Fifty effective individuals – the ICUN standard for avoiding inbreeding – equals a total population of 250 to 500. This means that, in a hypothetical apocalypse, humanity would need a lot more than a handful of survivors to repopulate effectively.

However, to retain evolutionary potential – to remain genetically flexible and diverse – the IUCN criteria suggest we would need at least 500 effective individuals. That requires a population of 2,500 to 5,000.

Some preliminary results emerging from ongoing research at the Centre of Excellence for Australian Biodiversity and Heritage appear to confirm this. Using both ancient DNA techniques and palaeo-demographic models, we have estimates of a minimum effective population size for Aboriginal Australians when they first appeared of about 250. This means at least several thousand had to arrive around the same time to manage to colonise the entire continent successfully.

Of course, not every species has the same ratio of effective to total population size, and not all populations necessarily need 5,000 individuals to survive. But without being able to measure the true ratio for a specific population, it helps to default to the average situation.

The idea that 50 individuals is enough to avoid inbreeding depression comes largely from laboratory populations that probably do not describe the situation for populations living in wild environments.

In species as varied as houseflies and pinkfairies, populations substantially greater than 50 individuals still succumb to inbreeding depression. So, in many cases, 50 effective individuals is in fact too low to ensure no inbreeding depression occurs. It may be that 100 effective individuals is closer to the true minimum, without even considering how populations respond to evolutionary challenges.

So, sensational analogies about the apocalypse aside, do human beings follow the same rule? We aren’t entirely sure, but evidence suggests that most species in vastly different groups roughly follow the same trend.

An emerging rule of thumb is that when a population starts to dip below several thousand individuals, it has a high likelihood of going extinct.

Conservation cost

The first issue is whether the cost of moving the rhinos is unjustified. The $4m cost is almost double the anti-poaching budget for South African National Parks ($2.2m), the managers of the estate where most white rhinos currently reside in the country.

The money would be better spent on anti-poaching activities in South Africa to increase local capacity. Or, from an Australian perspective, given the country’s abysmal record of mammal extinctions, it could go towards protecting indigenous species there.

In addition, there is the time cost of using the expertise of business leaders, marketeers and scientists. All could be working on conservation issues of much greater importance.

Bringing animals from the wild into captivity introduces strong selective pressure for domestication. Essentially, those animals that are too wild don’t breed and so don’t pass on their genes, while the sedate (unwild) animals do. This is exacerbated for species like rhinos where predation has shaped their evolution: they have grown big, dangerous horns to protect themselves. So captivity will likely be detrimental to the survival of any captive bred offspring should they be returned to the wild.

It is not known yet which rhino species will be the focus of the Australian project, but it will probably be the southern white rhino subspecies – which is the rhino species least likely to go extinct. The global population estimate for southern white rhinos (over 20,000) is stable, despite high poaching levels.

This number stands in stark contrast to the number of northern white (three), black (4,880 and increasing), great Indian (2,575), Sumatran (275) and Javan (up to 66) rhinos. These latter three species are clearly of much greater conservation concern than southern white rhinos.

There are also well over 800 southern white rhinos currently held in zoos around the world.

With appropriate management, the population size of the southern white is unlikely to lose genetic diversity, so adding 80 more individuals to zoos is utterly unnecessary. By contrast, across the world there are 39 other large mammalian herbivore species that are threatened with extinction that are far more in need of conservation funding than the five rhino species.

Exploitation

Rhinos inhabit places occupied by other less high profile threatened species – like African wild dogs and pangolins – which do not benefit from the same level of conservation funding. Conserving wildlife in their natural habitat has many benefits for the creatures and plants they coexist with. Rhinos are keystone species, creating grazing lawns that provide habitats for other species and ultimately affect fire regimes (fire frequency and burn patterns). They are also habitats themselves for a range of species-specific parasites. Abandoning efforts to conserve rhinos in their environment means these ecosystem services will no longer be provided.

Finally, taking biodiversity assets (rhinos) from Africa and transporting them to foreign countries extends the history of exploitation of Africa’s resources. Although well-meaning, the safe-keeping of rhinos by Western countries is as disempowering and patronising as the historical appropriation of cultural artefacts by colonial powers.

Conservation projects are ultimately more successful when led locally. With its strong social foundation, community-based conservation has had a significant impact on rhino protection and population recovery in Africa. In fact, local capacity and institutions are at the centre of one of the world’s most successful conservation success stories – the southern white rhino was brought back from the brink, growing from a few hundred in South Africa at the turn of the last century to over 20,000 throughout southern Africa today.

In our opinion, this project is neo-colonial conservation that diverts money and public attention away from the fundamental issues necessary to conserve rhinos. There is no evidence of what will happen to the rhinos transported to Australia once the poaching crisis is averted, but there seems nothing as robust as China’s “panda diplomacy” where pandas provided to foreign zoos remain the property of China, alongside a substantial annual payment, as do any offspring produced, for the duration of the arrangement.

With increased support, community-based rhino conservation initiatives can continue to lead the way. It is money that is missing, not the will to conserve them nor the expertise necessary to do so. Using the funding proposed for the Australian Rhino Project to support locally-led conservation or to educate people to reduce consumer demand for rhino horn in Asia seem far more acceptable options.

In Kaziranga, a national park in north-eastern India, rangers shoot people to protect rhinos. The park’s aggressive policing is, of course, controversial, but the results are clear: despite rising demand for illegal rhino horn, and plummeting numbers throughout Africa and South-East Asia, rhinos in Kaziranga are flourishing.

Yet Kaziranga, which features in a new BBC investigation, highlights some of the conflicts that characterise contemporary conservation, as the need to protect endangered species comes into contact with the lives and rights of people who live in and around the increasingly threatened national parks. India must balance modernisation and development with protections for the rights of local people – all while ensuring its development is ecologically sustainable.

To understand what’s at stake in Kaziranga, consider these three crucial issues:

1. The militarisation of conservation

The BBC feature shows park rangers who have been given the license to “shoot-on-sight”, a power they have used with deadly effect. In 2015 more than 20 poachers were killed – more than the numbers of rhino poached that year.

The programme accuses the rangers of extra-judicial killings of suspected rhino poachers. This resonates with a wider trend in the use of violence in defence of the world’s protected areas and the growing use of military surveillance technologies to support the efforts of conservation agencies.

In India, the Forest Department, which is responsible for the protection of wildlife sanctuaries and national parks, has always been a “uniformed” service. Rangers wear military-style khakis, are allowed to carry arms, and have powers to prosecute offenders. Recently, the government allowed them to use drones as an anti-poaching measure in Kaziranga.

To justify such escalation and its talk of a “war” against poaching, the government cites the growing power and sophistication of the crime syndicates involved in the illegal wildlife trade. However, as with all wars, a serious conflict over rhinos risks collateral damage. The worry is that increased militarisation is not conducted within strict legal limits or subject to judicial scrutiny. The BBC alleges that such checks and balances were not in place in Kaziranga.

2. The rights of local and indigenous populations

The BBC story also points to the growing conflict in and around Kaziranga between the interests and rights of local and indigenous people and the need to protect threatened species. Groups including Survival International – which features in the BBC story – claim that well-meaning conservation projects have denied and undermined the rights of indigenous groups around the world. The group calls for these rights to be placed at the heart of modern approaches to conservation – and most enlightened environmentalists now agree. It’s increasingly hard to look at conservation without also considering human rights and social issues.

Kaziranga is also home to tigers, elephants, buffalo and these swamp deer.kongsak sumano

Yet there remains significant tension between India’s wildlife conservation lobby, which perceives the Forest Rights Act as the death-knell for nature, and groups such as Survival International which argue that it is only by recognising the rights of local people that the country’s wildlife will be protected.

3. Can we keep expanding protected areas?

To protect threatened species across the world, conservationists have called for more and more land to be placed under protection. Renowned biologist EO Wilson, for instance, wants us to set aside “half the planet”.

In an unconstrained world, dedicating half the earth to the protection of the most threatened species and the world’s important habitats might seem like a sensible way to avoid the risks of what people fear might trigger the next great extinction. In reality, there are few places left where such a proposal might practically be implemented.

Perhaps nowhere is this more evident than in rapidly developing India. The country already has a population of 1.3 billion – and it aspires to both develop as a global economic powerhouse and lift its poorest people out of poverty. This development requires land and resources, with little space left for nature.

Plans to double the size of Kaziranga means villagers are being displaced with little due process and there are documented cases of violence and even death. This is a violent “green grab”, where land is usurped for ostensibly progressive environmental objectives, but which results in the dispossession of some of the most vulnerable people on this planet.

Kaziranga illustrates the dilemmas of contemporary conservation. If it is to be successful, environmentalism in India must be seen as part of the changing social and economic context, and not set itself up in opposition to these wider trends.

Conservation needs to recognise the need to build bridges, sometimes with its fiercest critics. While Kaziranga is in many ways a remarkable conservation success, its costs are considerable. The forces driving the world to overuse its resources haven’t gone away, and finding sustainable futures for both people and the planet requires coalitions that work together – let’s begin with Kaziranga.

Bhaskar Vira, Reader in Political Economy at the Department of Geography and Fellow of Fitzwilliam College; Director, University of Cambridge Conservation Research Institute, University of Cambridge